The hydrolysis reactions of many complex metal hydrides, including sodium borohydride (NaBH4), have been commonly used for the generation of hydrogen gas. The governing chemical reaction for such hydrolysis may be expressed as: where MBH4 and MBO2 respectively represent a metal borohydride and a metal metaborate. As the hydrolysis of sodium borohydride is typically slow at room temperature, heat or a catalyst, e.g., acids or a variety of transition metals, can be used to accelerate the hydrolysis reaction. The transition metals can include ruthenium, cobalt, nickel, or iron, or corresponding metal salts in solution or as solids, or metal borides as suspensions, or such salts, borides or metals can be deposited on inert supports. In addition, the rate of decomposition of the complex metal hydride into hydrogen gas and a metal metaborate is pH dependent, with higher pH values hindering the hydrolysis. Accordingly, solutions of a complex metal hydride (such as sodium borohydride), a stabilizer (such as sodium hydroxide (NaOH)), and water are used as the fuel from which the hydrogen gas is generated.
In those applications where a steady and constant supply of hydrogen is required, it is possible to construct hydrogen generation apparatus that controls the contact of a catalyst with the hydride fuel. Various hydrogen generation systems have been developed for the production of hydrogen gas from an aqueous sodium borohydride fuel solution (for example, U.S. patent application Ser. No. 09/900,625, “Portable Hydrogen Generator”, filed Jul. 6, 2001 and U.S. patent application Ser. No. 09/902,899, “Differential Pressure-Driven Borohydride Based Generator”, filed Jul. 11, 2001). These systems may be referred to as “two-tank” systems, where one tank is required for the storage of an aqueous metal borohydride solution and a separate tank is used to contain the discharged metal metaborate solution. A catalyst bed reactor connects the two tanks, and a fuel pump meters fuel solution to contact the catalyst bed. While these systems perform satisfactorily in many applications, they are larger than is desired for other applications and, in addition, require a source of power for the fuel pump.
A simple, one-tank recirculating system that uses pressure regulation is described in U.S. patent application Ser. No. 09/979,363, “A System for Hydrogen Generation,” filed Jan. 7, 2000. In this system, the catalyst bed is fixed in place and the pressure produced by generated hydrogen causes the solution to be forced away from the catalyst, thereby slowing down the hydrogen generation process. The generator described in this reference must remain upright in order to operate and to prevent unwanted contact of the fuel with the catalyst. In addition, if the demand for hydrogen ceases, the generator must either be drained or remain pressurized in order to prevent the fuel solution from contacting the catalyst bed.